Hamster polyomavirus (HaPV) is a small, circular DNA virus which belongs to the papovavirus family. HaPV was originally isolated from skin epitheliomas originating from hair follicle epithelial cells in Syrian golden hamsters. HaPV is most closely related to murine polyomavirus (Py), although it is distinguished from Py and other polyomaviruses in several ways. First, it is the only polyomavirus which naturally causes both epitheliomas and lymphomas in its native host, the Syrian golden hamster. Second, similar to the papillomaviruses, active viral replication with mature particle formation has only been detected in the differentiated layer of the epithelium in hamsters. Third, virus replication and tumorigenesis by the virus appears to be restricted exclusively to the Syrian golden hamster. The unique features of this virus make it an interesting virus to study, not only as a means to further understand the regulation of polyomavirus replication, but to understand the induction of leukemias in virus-infected animals.